Cyclic adsorption processes are frequently used to separate the components of a gas mixture, typically using one or more adsorber vessels that are packed with a particulate adsorbent material which adsorbs at least one gaseous component of the gas mixture more strongly than it adsorbs at least one other component of the mixture. The adsorption process comprises repeatedly performing a series of steps, the specific steps of the sequence depending upon the particular cyclic adsorption process being carried out. In any cyclic adsorption process, the adsorber bed has a finite capacity to capture a given gaseous component and therefore the adsorbent requires periodic regeneration to restore its adsorption capacity. The procedure followed for regenerating the adsorbent varies according to the process. In VPSA processes, the adsorbent is at least partially regenerated by creating a vacuum in the adsorber vessel thereby causing adsorbed components to be desorbed from the adsorbent. Also the adsorption step is carried out at a pressure higher than the regeneration step.
A typical VPSA process, such as detailed in U.S. Pat. No. 5,122,164 comprises a series of five basic steps that includes (i) Pressurization of the bed to the required pressure, (ii) Production of the product gas, (iii) Evacuation of the bed, (iv) Purging the bed with product gas under vacuum conditions and (v) Pressure equalization step to minimize vent losses and improve efficiency.
As mentioned above, the regeneration process includes a purge step during which a gas stream that is depleted in the component to be desorbed is passed counter-currently through the adsorber bed thereby reducing the partial pressure of adsorbed component, which in turn causes additional adsorbed component to be desorbed from the adsorbent. The non-adsorbed gas product may be used to purge the adsorber beds since this gas is usually quite depleted in the adsorbed component of the feed gas mixture. It often requires a considerable quantity of purged gas to adequately regenerate the adsorbent. For example, it is not unusual to use half of the non-adsorbed product gas produced during the previous production step to restore the adsorbent to the desired extent.
Many process improvements have been made to this simple cycle design in order to reduce power consumption, improve product recovery and purity, lower capital cost and increase product flow rate. These have included multi-bed processes, single column rapid pressure swing adsorption and more recently piston driven rapid pressure swing adsorption and radial flow rapid pressure swing adsorption. The trend toward shorter cycle times is driven by the desire to design more compact processes with lower capital costs and lower power requirements.
One of the improvements was made in U.S. Pat. No. 5,679,134 which suggested using a single bed with a reversible blower to reduce the complexity of the process. In U.S. Pat. No. 5,906,674 a tank was used to store low purity purge gas to improve the productivity of the process.
Investigation of these prior arts defined significant deficiencies which made them impractical for use in the industry both on small and large scale. The present invention, in distinction from the prior art, provides a safer, more practical and energy efficient process.